Darkness is coming down, but by degrees so infinitesimal and slow that eye and brain cannot measure its descent. Minute by minute, light drains from the wood. Yet still I can see, and if the deer move at all, this is the time at which they will come.

Perched on a high seat 15 feet off the ground, my back against an ash trunk, I have a panoramic view. Across my knees lies a .243 rifle, for I am trying to cull muntjac, or barking deer - the strange little creatures of Asiatic origin which have colonised much of the south of England.

Recent felling of fir has left this part of the forest pleasantly open. Visibility is good, but colours are fading: green of fir branches, russet of dead bracken, ochre of forest floor - all are turning to the "sober livery" of Milton's "twilight gray". While day clings on, I try again to memorise particular lumps: the single rock, the fallen tree trunk, the twisted elder that looks like a deer's head. These are my markers.

Earlier in the afternoon the wind blew viciously from the east; but now it has died, and in the stillness every sound rings out. Away to my left front a blackbird is mobbing something - probably an owl, for the call is the high, hysterical chatter aimed at airborne enemies, rather than the low tuk, tuk reserved for predators on the ground.

Movement is what I am looking for. With trees and plants motionless, any movement will take my eye. There - a pigeon sweeps in to roost, landing with a clatter of wings, while a grey squirrel out on some late errand flips along the springy upper branches of a larch.

To sit here in the wood is immensely therapeutic. My eyes and ears are on full alert, but my mind, lulled by the silence, winds down and reaches out across time. Perhaps 10,000 years have passed since the glaciers of the last ice age pulled back and carved this hill into its present form. Across my front, 50 yards out, run the ruins of an old stone wall. Old maps show the land beyond it as a field: now a dense mass of scrub and young trees covers it. It is from that thicket that I hope the muntjac will emerge.

These barking deer are relative newcomers on the scene. Their ancestors were imported by the eighth Duke of Bedford for his park at Woburn in 1894; but because they did not flourish in captivity, they were released into the wild, and their descendants have spread all over. Another immigrant, still less welcome, is the grey squirrel, brought over from North America in about 1860, and now the greatest menace to forestry in the kingdom.

Who can say what this hilltop will be like a hundred, two hundred, ten thousand years from now? With the onset of night comes a sense of human fallibility, of the transient nature of life on earth.

The day is nearly gone. Binoculars still bring objects into bold relief, but the telescopic sight on the rifle gathers light less efficiently, and I can hardly see the cross-hairs. To the naked eye, outlines are becoming blurred: my markers have lost their shapes and are merging into the background.

Then, at last, a definite movement at ground level. Some dark object is coming from right to left, along the line of the ancient wall. Up with the glasses. A fox, framed at one end by its creamy throat, at the other by the white tip of its brush. I watch it pause to sniff a low branch. Then, silent as a shadow, it passes on its way.

A tawny owl hoots. It is time to go. As I reach the foot of the tree, a single bark rips out from along the hill - a muntjac's alarm call or territorial challenge. Ten seconds later another bark echoes into the dark. The animal is too far off to have heard or scented me: it must be feuding with a rival. Nevertheless, I cannot help feeling that its harsh outburst incorporates a message for me as well - meaning, as it does, "Get lost!"